Interferometric hypsocline generator

ABSTRACT

A hypsocline or interference pattern having a shape and position representing the shape and position of an isoelevation area in a scene is produced from two phototransparent stereo images by directing mutually coherent collimated laser beams to strike the two stereo images. The images diffract the beams and thus transmit image information to the laser beams. The diffracted beams are directed to intersect, and any portions of the two intersecting beams representing the same area in the scene that are in registration with each other provide an interference pattern comprising alternate light and dark lines. The interference pattern has the shape and position of the represented area in the scene. The images are oriented with respect to each other so that the patterns produced represent isoelevation areas in the scene. A relative movement is provided between one stereo image and the beam striking that image to alter the portions in registration and thus provide different interference patterns representing areas at different elevations. The D.C. spatial frequency components are removed from the diffracted beams in order to maximize the difference between the interference patterns representing isoelevation areas and background signals surrounding those interference patterns.

United States Patent [191 Kowalski [45] July 31, 1973 INTERFEROMETRICHYPSOCLINE GENERATOR [75] Inventor: Daniel C. Kowalski, Wyandotte,

Mich.

[731 Assignee: The Bendix Corporation, Southfield,

Mich.

[22] Filed: June 22, 1971 [2]] Appl. No.: 155,459

Primary ExaminerRonald L. Wibert Assistant Examiner-F. L. EvansAtt0rney-J0hn S. Bell et al.

[57] ABSTRACT A hypsocline or interference pattern having a shape andposition representing the shape and position of an isoelevation area ina scene is produced from two phototransparent stereo images by directingmutually coherent collimated laser beams to strike the two stereoimages. The images diffract the beams and thus transmit imageinformation to the laser beams. The diffracted beams are directed tointersect, and any portions of the two intersecting beams representingthe same area in the scene that are in registration with each otherprovide an interference pattern comprising alternate light and darklines. The interference pattern has the shape and position of therepresented area in the scene. The images are oriented with respect toeach other so that the patterns produced represent isoelevation areas inthe scene. A relative movement is provided between one stereo image andthe beam striking that image to alter the portions in registration andthus provide different interference patterns representing areas atdifferent elevations. The DC. spatial frequency components are removedfrom the diffracted beams in order to maximize the difference betweenthe interference patterns representing isoelevation areas and backgroundsignals surrounding those interference patterns.

26 Claims, 6 Drawing Figures Pmimeum 3.7491492 SHET 1 0F 4 INVENTORDANIEL C. KOWALSKI ATTORNEY PATENTEBJUL 31 I975 SHEET 2 OF 4 FIG.2

Z 0 q x 5 m F X 6 0 w w P X, Z I x x m M -X x IN V EN TOR DANIEL CKOWALSK I ATTORNEY FIGS PAIENIEU JUL 31 ms SHEEI 3 OF 4 INVENTOR DANIELC. KOWALSKI ATTORNEY PATENTEDJULI? 1 I973 3. "(49 ,482

SHEET 4 0F 4 INVENTOR DANIEL C. KOWALSKI 1 INTERFEROMETRIC HYPSOCLINEGENERATOR CONFIRMATORY LICENSE TO UNITED STATES GOVERNMENT The inventionherein described was made in the course of a contract with theDepartment of the Air Force.

BACKGROUND OF THE INVENTION 1. Field of the Invention Photogrammetry,and more particularly, the determination of the elevation of areas in ascene from stereo images of that scene.

2. Brief Description of the Prior Art It is well known that two stereoimages of a scene, or in other words, two images of the scene taken fromdifferent vantage points, can be superimposed on each other so that theportions of the stereo images representing all areas in the scene at oneelevation will be in registration with each other, and so that allportions of the stereo images representing areas in the scene at allother elevations will not be in registration. That is, all conjugateimage points representing areas in the scene at one elevation will beprecisely aligned with each other, and all other conjugate points of thestereo images will be slightly offset from each other. The portions ofthe two stereo images that are not in registration with each other tendto cancel each other to produce a gray, blurred background signal. Theareas in registration do not cancel each other and thus provide anoutput that is generally of greater intensity than the other portions ofthe image of the scene produced by the superimposed stereo images.

one known device provides a contour map of a scene by superimposing twostereo images onto each other so that all points at one elevation are inregistration and recording the image produced by the superimposed stereoimages. The two stereo images are then moved relative to each other tobring all points at another elevation into registration and a recordingis made of the resulting image. A contour map is produced by formingimages representing the areas at all elevations of interest and by thenforming a composite of each of the recorded images. One drawback of thisdevice is that it is very difficult to automatically identify and recordthe portions of the two stereo images that are in registration with eachother because the difference in intensity between areas of the stereoimages in registration with each other and those not in registration maybe very small. That is, the intensity of various portions of the imageproduced by the superimposition of the stereo images depends on theimage detail or nature of the scene at various portions of the stereoimages. It is thus possible with some images for areas of thesuperimposed stereo images that are not in registration to produce amore intense signal than those that are in registration.

SUMMARY OF THE INVENTION The subject invention comprises a system forproviding an output image having a portion that represents oneisoelevation area in the scene and that can be easily distinguished fromall other portions of the output image. Several systems or preferredembodiments are illustrated herein that provide an output image thatincludes an interference pattern having an outline and positionrepresenting the outline and position of an isoelevation area in ascene. The interference pattern is surrounded by a gray backgroundsignal and can be easily distinguished from that background signal. Theillustrated embodiments provide the interference pattern by directingbeams of mutually coherent, collimated laer light to strike twotransparent stereo images of a scene. The images diffract the beams andthus transmit the image information to the laser beams. The diffractedbeams are directed to intersect. The portions of the two intersectingbeams that represent the same areas in the scene and that are inregistration with each other provide an interference pattern having anoutline representing the outline of the represented area. The portionsof the two intersecting beams that are not in registration with eachother provide a gray background signal. The stereo images are positionedwith respect to the beams so that the interference pattern produced bythe intersecting, diffracted beams represents the entire area in thescene at one elevation. Different patterns representing areas atdifferent elevations are produced by moving one stereo imageperpendicular to the beams striking that stereo image in order toprovide registration between portions of the intersecting beamsrepresenting different areas in the scene at different elevations.

The invention also encompasses means for removing at least one spatialfrequency component from the two stereo images, or from patternsrepresenting those images, in order to maximize the difference betweenthe portions of the output image representing an isoelevation area inthe scene and the portions of that image representing other areas in thescene. The systems illustrated herein include means for removing theD.C. spatial frequency components from the diffracted laser beams. Asused herein, the term D.C. spatial frequency component refers to thatspatial frequency component having zero line pairs per millimeter. Theterm spatial frequency is a measure of image detail and is customarilymeasured in line pairs per millimeter. The DC. spatial frequencycomponent provides equal contributions to all portions of the outputimage and thus obscures the difference between the portions of theoutput image representing the selected isoelevation area and the otherportions of the output image. In the systems illustrated herein,spherical lenses are positioned to receive the diffracted laser beamsrepresenting the two stereo images and to separate the spatial frequencycomponents of those beams by forming the Fourier transform of thosebeams. Spatial filters are positioned downstream from the sphericallenses to block the propagation of the DC. spatial frequency componentand thus eliminate that component from the diffracted beams. Theseparation between the remaining spatial frequency components is theneliminated by forming a second Fourier transform on the diffractedbeams. This second Fourier transform distributes the image informationacross the beams in an appropriate form so that conjugate areas of theintersecting beams in registration with each other will provide aninterference pattern that is easily distinguished from the backgroundsignal.

The interference patterns produced by the abovedescribed systemscomprise perspective representations of isoelevation areas in a scene.The invention therefore also includes a lens system for magnifying andminifying various patterns produced to convert the perspectiverepresentations to orthographic represenrations. In order to providethis conversion, all output patterns representing areas in the scene atelevations less than a preselected reference elevation are magnifled byamounts pfdpbrtional to theiiifferences between those lesserel'evations-and the refe'fen c e elevation. Similarly, all output patternsrepresenting areas in the scene at elevations greater than the referenceclevation are demagnified or minified by amounts proportional to thedifferences between those greater elevations and the referenceelevation. The difference in elevation between the elevation of anyselected area in the scene and a reference elevation is proportional tothe relative displacement of one of the stereo images with respect tothe other between the position of that stereo image at which a patternis produced representing the area at the reference elevation and theposition of that stereo image at which a pattern is producedrepresenting the area at the selected elevation. The lens system thusconverts perpsective representations to orthographic representations bymagnifying or demagnifying those perpsective representations by amountsdetermined by the relative dispiacement between the stereo imagesrequired to produce each pattern.

The invention also comprises means for providing a permanent record ofpatterns representing the areas of the scene at various elevations. Inone embodiment illustrated herein, the apparatus for providing thispermanent record comprises apparatus for scanning a photodetector acrossthe output image. The portion of the output image representing anisoelevation area in the scene is caused to flash on and off at apredetermined frequency while the background signal is held constant.The photodetector provides an electric output signal indicating thetemperal frequency of received optic signals. The photodetector isscanned across the output image and the position of the photodetector isrecorded to provide a permanent record of the contour of isoelevationareas in the scene whenever a signal having the predetermined tempera]frequency is detected.

The invention also encompasses apparatus for facilitating identificationof interference patterns representing isoelevation areas in a scene.This apparatus includes means for adjusting the spatial frequency of theinterference pattern in order to maximize the difference between theinterference pattern and the background signal.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects, features, andadvantages of the invention defined by the appended claims, will becomeapparent from a consideration of the following description and theaccompanying drawings in which:

FIG. I is a perspective. schmeatic view of one embodiment of thehypsocline generating apparatus of this invention;

FIG. 2 is a perspective view ofa scene that illustrates the relationshipbetween that scene and two stereo images of that scene;

FIG. 3 illustrates an interference pattern that is provided by theapparatus of FIG. 1 and that represents the area at one elevation in thescene of FIG. 2;

FIG. 4 illustrates a modified embodiment of the apparatus of FIG. 1 thatincludes magnifying apparatus for magnifying and minifying the patternsrepresenting areas in a scene to provide orthographic representations ofthose areas;

FIG. 5 illustrates the operation of the magnifying apparatus of thesystem of FIG. 4; and

FIG. 6 illustrates a modified embodiment of the apparatus of FIG. 1having means for automatically providing a permanent record or mapindicating the elevation of all areas in a scene.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates ahypsocline generator 10 having apparatus 12 for projecting twocollimated beams 14 and 16 of mutually coherent laser light,interferometric apparatus 18 for receiving those beams amd providing aninterference pattern having an outline and position corresponding to aperspective representation of the outline and position of the entirearea of a scene at one selected elevation. The interference pattern isprojected onto a display screen 20.

The apparatus 12 for generating collimated beams of mutually coherentlaser light is conventional and comprises a laser source 22 whichproduces a thin laser beam. That beam is expanded by an objective lens24 and collimated by a collimating lens 26. A cube beam splitter 28receives the laser light from lens 28 and provides the two collimatedbeams 14 and 16.

The interferometric apparatus 18 for receiving those beams and providingan output interference pattern comprises two identical photocarriages 30and 32 which hold phototransparent stereo images 34 and 36,respectively. Each photocarriage includes a mounting plate 38 forholding a stereo image. The mounting plate 38 is attached to an X axisscrew 40, which is in turn mounted to a Y axis screw 42. These screwsare driven by motive means 44 such as servo motors or handwheels whichthus move the stereo images 34 and 36.

As is illustrated in FIG. 2, the stereo images 34 and 36 comprisephotographs of a scene 46 taken from two different vantage points. Theillustrated scene 46 is a land mass, but it is understood that theinvention may be used to measure and determine elevations of variousportions of any scene or object including such things as models ofmachine tools. The photographs 34 and 36 are separated by a distance Bin FIG. 2. This distance is commonly called the base of the stereo imagepair. The X and Y image coordinates of the stereo images 34 and 36 arecommonly defined to be parallel to and perpendicular to the base line,respectively. This convention is followed herein. The stereo images 34and 36 are mounted in the mounting plates 38 of the photocarriages 30and 32 so that the X and Y axes of the two stereo images are parallel toeach other. To simplify notation in the explanation of this embodiment,the stereo images 34 and 36 are mounted so that the X axes of thosestereo images are parallel to the X axis of the hypsocline generator 10.

Collimated laser beam 14 illuminates image 34, and a mirror 48 directsbeam 16 to illuminate image 36. Beams 14 and 16 have sufficiently largediameters so that they illuminate substantially the entire area ofimages 34 and 36. Stereo images 34 and 36 diffract the laser beamstriking those images and thus transmit image information to thosebeams. All images have a D.C. spatial frequency component. The D.C.component does not carry any image detail information, but insteadprovides equal intensity contribution to all portions of the image. Thiscomponent thus obscures any image pattern produced representing anisoelevation area. The interferometric apparatus 18 thus includes meansfor eliminating the DC. spatial frequency component from the modulatedbeams 14 and 16 representing the stereo images 34 and 36. This meansincludes two spherical lenses 50 and 52 that are positioned to receivethe diffracted beams 14 and 16, and to form the Fourier transforms ofthose diffracted beams. In other words, lenses 50 and 52 order theindividual light rays comprising the diffracted beams 14 and 16according to the angle of arrival of each ray at the lens, and therebyspatially separate the spatial frequency image components of the stereoimages 34 and 36. The Fourier transformed beam 14 is directed by amirror 54 to strike a spatial filter 56, and Fourier transformed beam 16strikes a second spatial filter 58. Each spatial filter comprises atransparent glass piece 60 having an opaque disc 62 positioned to blockthe propagation of the DC. spatial frequency components of the Fouriertransformed beams to thereby eliminate those components from the beams.Two spherical lenses 64 and 66 are positioned downstream from thespatial frequency filters 56 and 58 respectively to remove the spatialseparation between the remaining spatial frequency components of thebeams 14 and 16, respectively. These lenses form second Fouriertransforms of those beams. The spatial distribution of each spatialfrequency component remaining in these beams is thus the same downstreamfrom lenses 64 and 66 as it was upstream from lenses 50 and 52.

The beams 14 and 16 intersect proximate a cube beam splitter 67 whichsuperimposes the two beams and projects them along direction 167 todisplay screen 20 and provides an output image 68 such as that shown inFIG. 3 on that screen. This output image 68 comprises a gray backgroundor noise signal 70 surrounding an interference pattern 72 having twoparts 74 and 76. An interference pattern such as the pattern 72 isproduced by any portions of the intersecting beams 14 and 16 thatrepresent the same areas in the scene 46 and that are in registrationwith each other. The portions of the beams 14 and 16 representing allother areas of the scene produce the background signal 70. The portionsof the beams 14 and 16 representing the same areas in the scene 46 willbe aligned when the portions of the stereo images 34 and 36 representingthose same areas in the scene are placed in the same correspondingpositions relative to the center of the beams 14 and 16. The stereoimages 34 and 36 are positioned with respect to the beams 14 and 16 sothat the intersecting beams provide an interference pattern 76representing the entire area in a scene 46 at one elevation 77.

The interference pattern 72 comprises alternate light and dark lines orinterference fringes 78 and 80. The spacing between the fringes, or inother words the width of those fringes, can be adjusted iwth an opticalflat mechanism 82. The mechanism 82 is positioned to receive the Fouriertransformed beam 16 having spatially separated spatial frequencycomponents and to deflect that received beam. The deflection of theFourier transformed beam 16 alters the relative spatial distribution ofthe spatial frequency components of beam 16. Because of this deflection,the phase of beam 16 downstream from lens 66 will be slightly differentfrom the phase of that beam upstream from lens 52. The phaserelationship between the intersecting beams 14 and 16 can thus bechanged by adjusting the optical flat mechanism 82. This phase changealters the spatial frequency of the interference pattern 74, or in otherwords alters the width of the individual fringes forming that pattern.It is possible to provide interference fringes of such a large widththat the entire pattern is either completely dark or completely light.The apparatus 82 includes a transparent piece of glass or optical flat84 which transmits received light but also deflects the transmittedlight by an amount depending on the angle between the optical flat 84and the beam 16 striking that flat. Optical flat 84 is mounted in aring-shaped inner housing 86 which is in turn mounted in a ringshapedouter housing 88. The inner housing is mounted so that it can be rotatedabout the Y axis of the system 10. The optical flat 84 is mounted sothat it can be rotated with respect to the inner housing about the Xaxis of the system 10. The position of optical flat 84 can thus beadjusted to provide any desired spacing between the interference fringesof the pattern 74.

In operation, two stereo images 34 and 36 of a scene are mounted in themounting plates 38 of the photocarriages 30 and 32, respectively. Thestereo images may be mounted in any manner as long as their coordinatesare substantially parallel. For illustration, they are mounted so thatthe X axes of the stereo images are parallel to the X axis of thehypsocline generator 10. The stereo images 34 and 36 are then alignedalong the Y direction. That is, the stereo images are positioned so thatthe portions of beam 14 representing opposite sides along the Y axis ofimage 34 will be superimposed on and aligned with the portions of beam16 representing the opposite Y axis sides of stereo image 36. The Y axisalignment of the stereo images is a preliminary orientation process.Once the stereo images are aligned, there is no need for any furthermovement along the Y axis. Collimated beams 14 and 16 ave sufficientwidth to illuminate the entire areas of stereo images 34 and 36simultaneously. The intersecting beams produce an interference patternon screen 20 representing the entire area in the scene at one elevation.The particular elevation represented is determined by the X axisorientation of the stereo images. Different output patterns representingthe areas in the scene 46 at different elevations are produced byproviding a relative movement between the stereo images 34 and 36 alongthe X axis of the system 10. This is accomplished for example by holdingimage 36 in a fixed position and moving image 34 along the X axis of thesystem 10. An output pattern representing an area of the scene at ahigher elevation is provided by moving stereo image 34 in the minus Xdirection. The minus X direction is the direction that stereo image 34would be moved when oriented as illustrated in FIG. 2 in order todecrease the base B. Similarly, an output pattern representing an areaata lower elevation is provided by moving stereo image in the plus Xdirection, or in other words the direction that would increase the baseB illustrated in FIG. 2. Output interference patterns having an outlinecorresponding to the outline of the area of a scene at any particularelevation can thus be readily produced by moving one of the stereoimages along the X axis of the system 10. The position of theinterference patterns representing various areas in the scene correspondto the positions of the represented areas. The output patterns providedby the system 10 thus indicate both the shape and position of areas at aparticular elevation. The spatial frequency at the output pattern, or inother words the width of the interference fringes, can be adjusted toprovide interference patterns having a maximum amount of contrast withthe background signal by tilting the optical flat 84.

The output interference patterns provided by the embodiment of FIG. 1are perspective representations of the areas at various elevations inthe scene. FlG. 2 illustrates a second hypsocline generator 90illustrated in FIG. 1 in that it includes a zoom lens system 92 forconverting perspective representations of isoelevation areas toorthographic representations of those areas. The intersecting beamssuperimpose upon each other are are both projected by the beam splitter67 along direction 167. Since the two beams are superimposed on eachother. the separation between the spatial frequency components of thosebeams can be eliminated by a single spherical lens 98 positioneddownstream from the beam splitter 94 to form a Fourier transform of thesuperimposed beams. The two spherical lenses 64 and 66 of the system 10are thus replaced by the single spherical lens 98. Lens 98 forms imagessuch as image 68 illustrated in FIG. 3. The images produced by lens 98define a plane 100. Lens 98 is positioned such that the image plane 100is an appropriate distance from the input to the zoom lens system 92 sothat the zoom lens system is capable of receiving and magnifying orminifying the interference patterns produced. Image 100 is reflectedonto the input of the zoom lens system 92 by a mirror 101. This mirroris included in the system to make hypsocline generator 9i more compact.The zoom lens system comprises an array of lenses 102 and a motorapparatus 194 for changing the spacing between the various lenses tothereby alter the degree of magnification or minification provided bythe lens array. The zoom lens system 92 is not shown in detail becausethese systems are well known and are described in a number of placedincluding Kingslake, R., The Development of the Zoom Lens, Journal ofthe SMPTE, Vol. 69 August 1960, pp. 534-644.

The manner in which the zoom lens system 92 operates is illustrated byFIG. 5, which demonstrates the differences between the various positionsat which points in a scene appear in a perspective view of that sceneand in an orthographic view of that scene. FIG. comprises a scene 106having portions both above and below an arbitrarily selected referenceplane 108. The positions at which points X and X in the scene 106 appearon a positive, perspective image 110 of that scene formed using a camerahaving a focal length f positioned at point 112 are indicated by raysE14 and 116, which project from the reference point 112 through thepoints X and X respectively. The points X and X appear in a positive,perspective image of the scene 106 at the positions X and Xrespectively, or in other words at the positions at which rays 114 and116 intersect the reference plane 108. in contrast, the points X and Xappear at the positions X and X respectively in an orthographic image ofthe scene 106. That is, they appear at the positions defined by theperpendicular projections of those points onto the reference plane 108.As FIG. 5 illustrates, points in a scene at elevations higher than areference elevation appear at a greater distance from the center of aperpsective image, or in other words from the normal projection of point122 onto image 110 and reference plane 108, than they do from the centerof an orthographic image of the scene. Conversely, points in the sceneat elevations lower than the reference elevation appear closer to thecenter of a perspective image than they do from the center of anorthographic image of the scene. The zoom lens mechanism 92 thusconverts each interference pattern which is a perspective representationof an isoelevation area in a scene to an orthographic representation ofthe area by multiplying all patterns representing areas at elevationshigher than an arbitrarily selected reference elevation by a factorproportional to the difference between the higher elevation and thereference elevation, and by minifying all patterns representing areashaving elevations less than the reference elevations by a factorproportional to the difference between the lesser elevation and thereference elevation. Expressed mathematically, the zoom lens apparatus92 multiplies each pattern by a factor of (H where:

H the distance between the plane of two positive, perspective, coplanarstereo images of a scene measured with respect to a selected referenceelevation in the scene; and

h, the elevation of a point j in the scene measured with respect to theselected reference elevation, which has a positive value for elevationsgreater than the reference elevation and a negative value for elevationslower than the reference elevation.

Interference patterns representing areas in the scene 46 at differentelevations are obtained by providing a relative movement between stereoimages 34- and 36 in a direction parallel to the X axis of those images.The relationship between relative movement of the stereo images and theelevations of areas represented by different patterns produced isprovided by the mathematic expression:

P f/ i/ j) where:

Ap relative X axis displacement of the stereo images; B the base of thestereo images; and f the focal length of the camera for forming thestereo images. The zoom lens system 92 thus multiplies each interferencepattern representing an isoelevation area in the scene by a factor whichis determined by the relative displacement of stereo images 34 and 36required in order to provide that pattern. The displacement of images 34and 36 is measured with respect to the positions of those two stereoimages at which an interference pattern representing the area in thescene at a reference elevation is produced. This factor is expressedmathematically by solving equation 2 for h, and substituting theresulting expression into factor 1 to obtain:

1 +HAp/(Bf+ HAp) where: All symbols are as previously defined.

The hypsocline generating system also includes a recording film 118 forrecording the various interference patterns produced. Each of theinterference patterns produced by the system 90 is recorded on one frameof the film 118. The film is held by a film frame or holder 120 and ismoved by a motor apparatus (not shown) from a first light-tight rollercan 122 containing unexposed film to a second light-tight roller can 124which contains the exposed film. The exposed film frames can besubsequently processed and superimposed upon each other to obtain anorthographic hypsocline map of the scene which indicates the relativeelevation of all areas in that scene.

FIG. 6 illustrates a hypsocline generating system 130 that differs fromthe systems 10 and 90 in that it includes a computer controlledapparatus for automatically providing a permanent record of the area ateach elevation of the scene represented by the stereo images 34 and 36.A computer control 132 controls the operation of motors 44 and thuscontrols the position of the stereo images 34 and 36. Computer control132 also controls operation of motor 104 and thereby controls themagnification and minification provided by zoom lens array 102. The zoomlens array 102 provides output images which define a plane 134. Aphotoelectric detector 136 for receiving optic input signals andconverting those signals to electric output signals is scanned acrossplane 134. The apparatus for scanning detector 136 across plane 134comprises a computer controlled carriage 138 which is similar to thephotocarriages 30 and 32 holding stereo images 34 and 36. Signals aretransmitted from the computer control 132 to the motors 44 of carriage138 which cause the photodetector 136 to be scanned across plane 134.

In order to provide output interference patterns representing areas atvarious elevations in a scene that can be recognized and distinguishedfrom the background signals surrounding that interference pattern by thephotodetector 136 and the associated signal processing apparatus, thesystem 130 includes a retardation type laser light modulator 140 forperiodically eliminating the coherence between beams 14 and 16. Source22 provides a beam of polarized laser light. modulator 140 eliminatesthe coherence between beams 14 and 16 by rotating the polarity directionof beam 14 at a predetermined frequency. Since beams 14 and 16 areproduced by the same source, they are polarized in the same direction.When the direction of polarization of beam 14 is rotated by modulator140, the coherence between the two beams is destroyed so that they willno longer provide an interference pattern. Modulator 140 has no effecton the gray background signals surrounding an interference pattern. Theperiodic rotation of beam 14 thus causes the interference pattern inplane 134 representing an isoelevation area in the scene to flash on andoff. Commercially available modulators such as modulator 140 are capableof receiving only a narrow laser beam. Therefore, to accommodate thislimitation of the modulator, the objective and collimating lenses 24 and26 of the systems 10 and 90 are replaced by separate objective andcollimating lenses 142 and 144, respectively, placed in each of thebeams 14 and 16. Modulator 140 thus receives beam 14 before that beam iscollimated.

A high voltage A.C. electric signal for driving modulater 140 isprovided by a high voltage signal generator 142. Modulator 140 rotatesthe polarity of beam 14 at a frequency equal to the frequency of thehigh voltage A.C. signal provided by generator 142 and thus causes theinterference pattern produced in plane 134 to flash on and ofi at thefrequency of this A.C. signal. The signal from generator 142 is alsotransmitted to an electronic multiplier and filter apparatus 144.Apparatus 144 also receives the electric output signal fromphotodetector 136. Apparatus 144 compares the signal received fromvoltage generator 142 and photodetector 136 provides an output signalwhose value is proportional to the correlation between the frequenciesof the two received signals. The output from apparatus 144 istransmitted to a threshold detector 146 which in turn provides an outputto a motor 148 whenever a signal is received from apparatus 144 having avalue greater than a predetermined value computed by the controlcomputer 132. The tracing motor 148 causes that motor to extend atracing pencil 150 to strike a tracing surface 152 and thus trace alongthat surface whenever photodetector 136 receives a signal oscillating atthe frequency of the signal provided by high-voltage generator 142 andis thus aligned with an interference pattern representing anisoelevation area in the scene. Motor 148 retracts tracing pencil 150from surface 152 whenever photodetector 136 is spaced from such aninterference pattern and thus receives only background signals. Tracingmotor 148 and pencil 150 are mounted to move with detector 136 andthereby provide a record of the positions at which a flashinginterference pattern is detected.

In operation, stereo images 34 and 36 are sequentially positioned toprovide output images representing the area at each elevation in thescene. Detector 136 is scanned across each sequentially provided outputimage. Tracing pencil 150 provides a trace of the position of detector136 whenever that detector is aligned with an interference patternrepresenting an isoelevation area, and thereby provides a map 154indicating the contour of the scene represented by stereo images 34 and36.

Having thus described several embodimens of this invention, a number ofmodifications will occur to those skilled in the art.

Therefore, what is claimed is:

1. An interferometric method for determining the outline and position ofthe entire area of a scene at one elevation from substantially coplanarfirst and second stereo images representing the scene from first andsecond vantage points respectively, each image having a first axisparallel to a straight line connection between the vantage points, themethod comprising the steps of:

positioning the stereo images with the first axes substantially parallelto each other;

forming first and second wave energy signals that represent saidpositioned first and second stereo images respectively, and thatinterfere when superimposed on each other;

superimposing said first and second signals, the positioning of saidstereo images with said first axes parallel to each other causing theportions of said superimposed signals representing the entire area ofthe scene at one elevation ot register with each otehr and provide areadily recognizable interference signal representing the outline andposition of said area at said one elevation.

2. The method of claim 1 in which:

the particular elevation represented by the interference signal isdetermined by image position along a dimension parallel to said firstaxes; and

the method further includes the step of providing a relative movementbetween said first and second signals along a dimension corresponding tothe dimension of said substantially parallel first axes to sequentiallybring the portions of said signals representing areas of said scene atdifferent elevations into registration with each other and therebyprovide different interference signals representing the outlines andpositions of areas at different elevations. 3. The method of claim 1 inwhich: said first and second wave energy signals each have a D.C.spatial frequency component; and

the method further includes the step of removing said D.C. spatialfrequency components from said first and second signals.

4. The method of claim 1 further including the step of adjusting thespatial frequency of said interference signal to facilitate recognitionof said interference signal.

5. The method of claim 4 wherein:

said first and second wave energy signals comprise first and secondmutually coherent beams of laser light modulated to represent said firstand second stereo images respectively;

said interference signal comprises an optic pattern of alternate lightand dark lines; and

said adjusting of the spatial frequency of said interference patterncomprises altering the width of said lines.

6. The method of claim 1 further including the step pf periodicallyaltering said interference signal to facilitate recognition of saidinterference signal.

7. The method of claim 6 in which said altering comprises causing saidinterference signal to flash on and off.

8. The method of claim 2 in which:

said interference signals comprise optic patterns that defineperpsective representations of isoelevation areas of said scene; and

the method further includes the step of converting said perspectiverepresentations to orthographic representations.

9. The method of claim 8 in which said converting comprises magnifyingall output patterns representing areas in said scene having an elevationless than a preselected elevation by amounts proportional to thedifferences between said lesser elevations and said preselectedelevation, and demagnifying all output patterns representing areas insaid scene having elevations greater than said preselected elevation byamounts proportional to the differences between said greater elevationsand said preselected elevation to thereby provide said orthographicrepresentation.

10. The method of claim 1 further including the step of providing apermanent record of the outline and position of said area at oneelevation.

11. The method of claim 1 in which said forming of said first and secondsignals comprises:

directing first and second beams of mutually coherent, collimatedradiation to strike and be modulated by said first and second stereoimages respectively; and

the method further includes the steps of:

forming the Fourier transform of said modulated first and second beamsto thereby spatially separate the spatial frequency image components ofsaid first and second modulated beams;

eliminating the D.C. spatial frequency components from said Fouriertransformed first and second beams; and

forming a second Fourier transform of the remaining components of saidmodulated beams to thereby eliminate said spatial separation, saidsecond Fourier transform providing output beams in which the pattern ofeach spatial frequency components of said output beams is identical tothe pattern of said each spatial frequency components of said two stereoimages.

12. An interferometric system for determining the outline and positionof the entire area of a scene at one elevation from two substantiallycoplanar stereo images representing the scene from different vantagepoints, said images each having a first axis parallel to a straight lineconnection between the vantage points, said system comprising:

image holding means for holding the two stereo images in a spaced apartrelationship with the first axes of said images substantially parallelto each other;

first beam directing means for directing a first beam of coherentradiation to strike and be modulated by one of said stereo images;

second beam directing means for directing a second beam of radiationmutually coherent with said first beam to strike and be modulated by theother of said stereo images and then intersect said modulated firstbeam, the holding of said images with said first axes parallel to eachother causing the portions of said intersecting beams representing theentire area of said scene at one elevation to register with each otherand provide an interference pattern having an outline and positionrepresenting the outline and position of said area at said oneelevation.

13. The system of claim 12 in which:

the particular elevation represented by the interfering portions of saidbeams depends on the relative positions of said stereo images along adimension parallel to said first axes; and

said image holding means include means for moving one image with respectto the other along said dimension to change the portions of said beamsin registration with each other and thereby provide different patternsrepresenting different elevations.

14. The system of claim 13 wherein:

said first and second beams are mutually coherent beams of laser light;

said stereo images are partially transparent images that modulatereceived laser light by diffraction;

said interference pattern comprises a hypscoline; and I said second beamdirecting means includes means for directing said second beam to strikesaid other stereo image along a path that does not intersect themodulated first beam and means for deflecting the modulated second beamto intersect the modulated first beam.

15. The system of claim 12 further including means for eliminating atleast one preselected spatial fre- 0 quency component representing atleast one preselected degree of image detail from said first and secondmodulated beams.

16. The system of claim 15 in which said spatial frequency eliminatingmeans comprises means for eliminating the D.C. spatial frequencycomponent from said first and second modulated beams.

17. The system of claim 15 in which said spatial frequency eliminatingmeans comprises:

means for spatially separating the spatial frequency image components ofsaid first and second modulated beams representing different degrees ofimage detail;

spatial filter means for eliminating one of said separated spatialfrequency components from said first and second modulated beams; and

means for eliminating said spatial separation between the remainingspatial frequency components of said modulated beams to thereby providea distribution of image information corresponding to the distribution ofimage information on said two stereo images.

18. The system of claim 17 in which:

said means for spatially separating image components comprises means forforming the Fourier transform of said first and second modulated beans;and

said means for eliminating said spatial separation comprises means forforming the Fourier transform of said Fourier transformed, modulatedbeams.

19. The system of claim 18 in which:

said stereo images are partially transparent and said modulationcomprises diffraction of said first and second beams;

said beams are beams of mutually coherent beams of laser light;

said means for spatially separating image components comprises firstlens means for forming the Fourier transform of said modulated firstbeam positioned between said first image and the position at which saidbeams intersect, and second lens means for forming the Fourier transformof said modulated second beam disposed between said second image and theposition at which said beams intersect; and

said means for eliminating said spatial separation comprise third lensmeans for forming the Fourier transform of said modulated first andsecond beams.

20. The system of claim 19 further including:

means for projecting said first and second Fourier transformed,modulated beams along substantially the same path; and

said means for eliminating said spatial separation comprises a singlespherical lens disposed to receive said Fourier transformed modulatedbeams from said projecting means.

21. The system of claim 12 in which said interference pattern comprisesa cyclically varying intensity pattern, and said system includes meansfor altering the phase of said first beam with respect to said secondbeam to thereby alter the spatial frequency of said interferencepattern.

22. The system of claim 21 in which:

said system includes means for forming the Fourier transform of saidmodulated first beam to spatially separate the spatial frequencycomponents of modulated first beam;

said phase altering means comprises means for defleeting said Fouriertransformed first beam to vary the spatial distribution of the spatialfrequency components of said Fourier transformed first beam with respectto said second beam; and

said system includes means for eliminating said spatial separation ofsaid spatial frequency components of said first beam before said firstbeam intersects with said second beam.

23. The system of claim 13 in which:

said interference patterns comprise perspective representations ofisoelevation areas; and

said system includes magnifying means responsive to said relativemovement between said one image and said first beam for magnifying eachpattern produced by said system by a factor 1 HAp/(Bf-i-HAp) to convertsaid perspective representation to an orthographic representation where:

B a constant equal to the distance between the two vantage points;

f focal length of a camera for providing the two stereo images;

H distance between the plane of said coplanar stereo images and apreselected elevation; and

Ap displacement of said one stereo image along said dimension betweenthe position at which a pattern representing the area at saidpreselected elevation is produced and the position at which said eachpattern is produced.

24. The system of claim 12 in which:

said first and said second beams are polarized in substantially the samedirection; and

said system further includes means for rotating the polarity directionof said modulated first beam to periodically destroy said mutualcoherence and thereby cause said patterns to flash on and off, saidflashing facilitating recognition of said pattern.

25. The system of claim 24 further including means for providing apermanent record of the outline and position of said area at oneelevation comprising:

detector means for receiving said pattern, said detector means providingan output signal indicating the temperal frequency of receivedradiation;

means for identifying the position at which signals having a frequencysubstantially equal to the temperal frequency at which the polaritydirection of said first beam is changed from said polarity direction ofsaid second beam; and

recording means responsive to said position determining means forproviding a record of said determined positions.

26. The system of claim 25 in which:

said first and second beams comprise beams of laser light;

said interference pattern defines a plane;

said detector means comprises a photoelectric detector device, and meansfor scanning said detector device across said plane along apredetermined raster pattern; and

said position identifying means comprise means for identifying theposition of said detector device in said plane.

1. An interferometric method for determining the outline and position of the entire area of a scene at one elevation from substantially coplanar first and second stereo images representing the scene from first and second vantage points respectively, each image having a first axis parallel to a straight line connection between the vantage points, the method comprising the steps of: positioning the stereo images with the first axes substantially parallel to each other; forming first and second wave energy signals that represent said positioned first and second stereo images respectively, and that interfere when superimposed on each other; superimposing said first and second signals, the positioning of said stereo images with said first axes parallel to each other causing the portions of said superimposed signals representing the entire area of the scene at one elevation ot register with each otehr and provide a readily recognizable interference signal representing the outline and position of said area at said one elevation.
 2. The method of claim 1 in which: the particular elevation represented by the interference signal is determined by image position along a dimension parallel to said first axes; and the method further includes the step of providing a relative movement Between said first and second signals along a dimension corresponding to the dimension of said substantially parallel first axes to sequentially bring the portions of said signals representing areas of said scene at different elevations into registration with each other and thereby provide different interference signals representing the outlines and positions of areas at different elevations.
 3. The method of claim 1 in which: said first and second wave energy signals each have a D.C. spatial frequency component; and the method further includes the step of removing said D.C. spatial frequency components from said first and second signals.
 4. The method of claim 1 further including the step of adjusting the spatial frequency of said interference signal to facilitate recognition of said interference signal.
 5. The method of claim 4 wherein: said first and second wave energy signals comprise first and second mutually coherent beams of laser light modulated to represent said first and second stereo images respectively; said interference signal comprises an optic pattern of alternate light and dark lines; and said adjusting of the spatial frequency of said interference pattern comprises altering the width of said lines.
 6. The method of claim 1 further including the step pf periodically altering said interference signal to facilitate recognition of said interference signal.
 7. The method of claim 6 in which said altering comprises causing said interference signal to flash on and off.
 8. The method of claim 2 in which: said interference signals comprise optic patterns that define perpsective representations of isoelevation areas of said scene; and the method further includes the step of converting said perspective representations to orthographic representations.
 9. The method of claim 8 in which said converting comprises magnifying all output patterns representing areas in said scene having an elevation less than a preselected elevation by amounts proportional to the differences between said lesser elevations and said preselected elevation, and demagnifying all output patterns representing areas in said scene having elevations greater than said preselected elevation by amounts proportional to the differences between said greater elevations and said preselected elevation to thereby provide said orthographic representation.
 10. The method of claim 1 further including the step of providing a permanent record of the outline and position of said area at one elevation.
 11. The method of claim 1 in which said forming of said first and second signals comprises: directing first and second beams of mutually coherent, collimated radiation to strike and be modulated by said first and second stereo images respectively; and the method further includes the steps of: forming the Fourier transform of said modulated first and second beams to thereby spatially separate the spatial frequency image components of said first and second modulated beams; eliminating the D.C. spatial frequency components from said Fourier transformed first and second beams; and forming a second Fourier transform of the remaining components of said modulated beams to thereby eliminate said spatial separation, said second Fourier transform providing output beams in which the pattern of each spatial frequency components of said output beams is identical to the pattern of said each spatial frequency components of said two stereo images.
 12. An interferometric system for determining the outline and position of the entire area of a scene at one elevation from two substantially coplanar stereo images representing the scene from different vantage points, said images each having a first axis parallel to a straight line connection between the vantage points, said system comprising: image holding means for holding the two stereo images in a spaced apart relationship with the first axes of said images substantiallY parallel to each other; first beam directing means for directing a first beam of coherent radiation to strike and be modulated by one of said stereo images; second beam directing means for directing a second beam of radiation mutually coherent with said first beam to strike and be modulated by the other of said stereo images and then intersect said modulated first beam, the holding of said images with said first axes parallel to each other causing the portions of said intersecting beams representing the entire area of said scene at one elevation to register with each other and provide an interference pattern having an outline and position representing the outline and position of said area at said one elevation.
 13. The system of claim 12 in which: the particular elevation represented by the interfering portions of said beams depends on the relative positions of said stereo images along a dimension parallel to said first axes; and said image holding means include means for moving one image with respect to the other along said dimension to change the portions of said beams in registration with each other and thereby provide different patterns representing different elevations.
 14. The system of claim 13 wherein: said first and second beams are mutually coherent beams of laser light; said stereo images are partially transparent images that modulate received laser light by diffraction; said interference pattern comprises a hypscoline; and said second beam directing means includes means for directing said second beam to strike said other stereo image along a path that does not intersect the modulated first beam and means for deflecting the modulated second beam to intersect the modulated first beam.
 15. The system of claim 12 further including means for eliminating at least one preselected spatial frequency component representing at least one preselected degree of image detail from said first and second modulated beams.
 16. The system of claim 15 in which said spatial frequency eliminating means comprises means for eliminating the D.C. spatial frequency component from said first and second modulated beams.
 17. The system of claim 15 in which said spatial frequency eliminating means comprises: means for spatially separating the spatial frequency image components of said first and second modulated beams representing different degrees of image detail; spatial filter means for eliminating one of said separated spatial frequency components from said first and second modulated beams; and means for eliminating said spatial separation between the remaining spatial frequency components of said modulated beams to thereby provide a distribution of image information corresponding to the distribution of image information on said two stereo images.
 18. The system of claim 17 in which: said means for spatially separating image components comprises means for forming the Fourier transform of said first and second modulated beans; and said means for eliminating said spatial separation comprises means for forming the Fourier transform of said Fourier transformed, modulated beams.
 19. The system of claim 18 in which: said stereo images are partially transparent and said modulation comprises diffraction of said first and second beams; said beams are beams of mutually coherent beams of laser light; said means for spatially separating image components comprises first lens means for forming the Fourier transform of said modulated first beam positioned between said first image and the position at which said beams intersect, and second lens means for forming the Fourier transform of said modulated second beam disposed between said second image and the position at which said beams intersect; and said means for eliminating said spatial separation comprise third lens means for forming the Fourier transform of said modulated first and second beams.
 20. The system of claim 19 furtHer including: means for projecting said first and second Fourier transformed, modulated beams along substantially the same path; and said means for eliminating said spatial separation comprises a single spherical lens disposed to receive said Fourier transformed modulated beams from said projecting means.
 21. The system of claim 12 in which said interference pattern comprises a cyclically varying intensity pattern, and said system includes means for altering the phase of said first beam with respect to said second beam to thereby alter the spatial frequency of said interference pattern.
 22. The system of claim 21 in which: said system includes means for forming the Fourier transform of said modulated first beam to spatially separate the spatial frequency components of modulated first beam; said phase altering means comprises means for deflecting said Fourier transformed first beam to vary the spatial distribution of the spatial frequency components of said Fourier transformed first beam with respect to said second beam; and said system includes means for eliminating said spatial separation of said spatial frequency components of said first beam before said first beam intersects with said second beam.
 23. The system of claim 13 in which: said interference patterns comprise perspective representations of isoelevation areas; and said system includes magnifying means responsive to said relative movement between said one image and said first beam for magnifying each pattern produced by said system by a factor 1 + H Delta p/(Bf+H Delta p) to convert said perspective representation to an orthographic representation where: B a constant equal to the distance between the two vantage points; f focal length of a camera for providing the two stereo images; H distance between the plane of said coplanar stereo images and a preselected elevation; and Delta p displacement of said one stereo image along said dimension between the position at which a pattern representing the area at said preselected elevation is produced and the position at which said each pattern is produced.
 24. The system of claim 12 in which: said first and said second beams are polarized in substantially the same direction; and said system further includes means for rotating the polarity direction of said modulated first beam to periodically destroy said mutual coherence and thereby cause said patterns to flash on and off, said flashing facilitating recognition of said pattern.
 25. The system of claim 24 further including means for providing a permanent record of the outline and position of said area at one elevation comprising: detector means for receiving said pattern, said detector means providing an output signal indicating the temperal frequency of received radiation; means for identifying the position at which signals having a frequency substantially equal to the temperal frequency at which the polarity direction of said first beam is changed from said polarity direction of said second beam; and recording means responsive to said position determining means for providing a record of said determined positions.
 26. The system of claim 25 in which: said first and second beams comprise beams of laser light; said interference pattern defines a plane; said detector means comprises a photoelectric detector device, and means for scanning said detector device across said plane along a predetermined raster pattern; and said position identifying means comprise means for identifying the position of said detector device in said plane. 